In internal combustion engines, a process known as exhaust gas recirculation ("EGR") is used to reduce NO.sub.x emissions. In general, EGR involves routing a portion of exhaust gas back into the intake air flow. In an engine wherein exhaust backpressure is greater than the intake air pressure (e.g., most normally aspirated engines) an EGR flow can be realized simply by connecting a conduit between the exhaust and the intake ducts. Because of the negative pressure differential, the flow from the exhaust manifold is drawn to the lower pressure of the intake. However, in an engine having a charged intake, an unfavorable pressure differential must be overcome.
More particularly, in turbocharged diesel engines, the recirculated exhaust gas flow is typically introduced into the intake downstream of turbo-compressor and intercooler components in order to avoid degradation of these components. Unfortunately, the intake air is pressurized at this location, presenting an unfavorable intake-to-exhaust manifold pressure ratio for transporting EGR gases. Diesel engines with efficient, well-matched turbochargers have an insufficient exhaust-to-intake manifold pressure differential during virtually all operating states to induce useful amounts of EGR.
Various EGR systems are known which attempt to overcome this unfavorable pressure differential. One known system provides a pump or compressor that creates a pressure differential sufficient to force the desired exhaust-to-intake flow from the exhaust stack into the intake manifold. However, this requires a significant amount of energy to pump the exhaust from a low or near-zero gauge pressure up to more than the intake manifold pressure. As a result, these known devices tend to substantially sacrifice fuel economy.
An EGR system for a turbocharged engine is disclosed in U.S. Pat. No. 5,564,275 in which exhaust gas flow is drawn from upstream of the exhaust turbine and pumped to the intake by a separate EGR turbocompressor that is driven by a separate branch of the exhaust. A rate of EGR flow is adjustable by a control valve located in that exhaust flow branch driving the EGR turbine. This exhaust-driven type of EGR compressor system can have a response lag from spooling the EGR turbocompressor. Moreover, such a system produces an EGR flow which is limited in capacity, because its pumping capacity is directly dependent on the engine exhaust flow rate.
A need exists for an EGR system which is fuel efficient and which is capable of producing a desired amount of EGR flow on demand.